Progress in the development of photovoltaic cells depends on a variety of factors, not least of which are new designs, new materials and new fabrication techniques. Historically, much effort is rightfully placed on attempting to increase the solar conversion efficiency. Progress has been dramatic. For AM1 illumination (sunlight through one thickness of the earth's atmosphere), a selenium solar cell in 1914 had an efficiency of 1%, by 1954 an efficiency of 6% was achieved for a silicon single-crystal cell, whereas by the mid-1980's efficiencies of between 22-25% were reported in solar cells. With concentrator cells, where lenses or mirrors are used to increase sunlight to considerably greater than normal intensity, efficiencies of 27.5% have been reported, which compares favorably with the 38-40% thermal efficiency in a typical fossil fuel power plant and the 32-34% efficiency of a light-water nuclear reactor power plant.
To make a solar cell economical for large-scale applications, however, such as for providing electrical power to residences, other considerations besides high efficiency are prominent. One factor is the fabrication cost of a cell. While most detached homes have enough roof area for solar cells of conventional design to provide 8500 kW-hrs of electricity annually, which is sufficient for the average home, one bottleneck to commercialization is not efficiency but lowering the costs per unit area of a solar cell. A promising candidate for this task is silicon solar cells, especially those cells fabricated from thin (.about.100 .mu.m) silicon substrates where high-quality silicon is effectively utilized. The challenge at present is to decrease the unit costs for these solar cells so that they may be competitive with traditional fossil fuel power supplies at present energy prices. One way to do this is through improved fabrication techniques.
In addition to fabrication techniques, certain design structures offer advantages over other designs. One such superior design seems to be back contact solar cells, in particular those employing thin silicon substrates.
Homojunction silicon solar cells have a p-n junction for separating photogenerated electrons from photogenerated holes. For the solar cell to function properly, electrons must be directed toward the contact for the n-type material and holes must be directed toward the contact for the p-type material. Light intensity in a semiconductor decreases monotonically with depth, thus the p-n junction is preferably close to the illuminated surface, to reduce recombination of holes and electrons, prior to their being separated by the p-n junction. In thin silicon solar cells, though the thickness of a cell is smaller than in conventional silicon solar cells (.about.300 .mu.m), and the probability of a photon being converted into an electron-hole or charge-carrier pair is less, the average lifetime of a photogenerated electron-hole pair can be such that the photogenerated electron-pair will survive being swept to their respective contacts. That is to say, in a thin silicon solar cell the minority carrier diffusion length can be relatively large compared to the thickness of the cell so performance of the cell is not unduly compromised. In the present invention the minority carrier diffusion length is equal to the thickness of the cell or greater.
Further, conventional (front-contact) silicon solar cells have a structure in which a large p-n junction is formed over the entire substrate on the illuminated side of the cell. This conventional design has the virtue of simplicity, in that no patterning is required for the emitter (typically the p-type layer in a p-n junction cell) since it covers the entire front surface. However, simultaneous and conflicting requirements are imposed on the front surface and the emitter layer in this type arrangement. On the one hand, the emitter diffusion should be shallow and have a low dopant concentration (&lt;1.times.10.sup.19 cm.sup.-3) in order to reduce recombination, which occurs with higher dopant concentrations. On the other hand, such a shallow, lightly-doped emitter will have a high sheet resistance (current flows laterally through the top layer of a conventional cell, and in between any contact grid lines, and sheet resistance is inversely proportional to the doped layer thickness), generally greater than 100 ohms/square, which would necessitate that grid contact lines be closely spaced to avoid excessive ohmic power losses.
Closely spaced contact lines in a conventional front-contact cell means reduced power from the cell due to shadowing of the underlying silicon by the contact material. In addition, if the dopant concentration is low, the contact-dopant layer interface will be rectifying (like a Schottky diode) rather than be ohmic, with a corresponding power loss associated with the turn-on voltage of the diode. But the higher the dopant concentration, the greater the recombination of electrons and holes in the emitter layer, which is deleterious and typically occurs greatest near the surface where incoming light shines. Finally, texturing of the front surface to increase light trapping means contact lines have to run over a rough surface without loss of continuity, which can be difficult to achieve. In addition, some texturing methods, such as the porous silicon method, will make creating an emitter diffusion layer of acceptable uniformity more difficult.
For this reason and others, for a conventional cell structure a balance must be sought between the desirability for a heavily-doped surface to promote ohmic contact formation and reduced shadowing and the desirability of a lightly-doped surface for reduced carrier recombination and effective surface passivation. Constraints due to texturing and shadowing are also a problem. An alternative approach is to place the p-n junction on the back (non-illuminated side) of the cell. In such a back-contact solar cell the requirements for texturing and passivating the front surface are separated from the requirements for forming the p-n junction and for contacting the emitter and the base. This means the p-n junction can be deep and the emitter can be heavily doped without extreme consequences. Shadowing of the illuminated surface is no longer an issue since there are no contacts on the front surface, and neither is the spacing of the metal contact lines a problem. Since this type of cell generally employs interdigitated contacts, nearly half the back surface area is covered with positive contact metal and the other half is covered with negative contact metal. Because the p-n junction is on the back of the cell, however, the minority carrier diffusion length in the starting material (base) must exceed the cell thickness in order to obtain satisfactory energy conversion efficiency. The best results for this approach are from a Stanford University group, which has reported efficiencies of 21.3% at one sun (100 mW/cm.sup.2) illumination on a float-zone back-contact silicon cell 180 .mu.m thick and 35 cm.sup.2 in area; and 22% for one sun AM1 illumination at 24.degree. C. (R. A. Sinton et al., "Large-Area 21% Efficient Si Solar Cells" , Conf. Record 23rd IEEE Photovoltaic Specialists Conference, p. 157 (1993); R. A. Sinton et al., IEEE Electron Device Lett., EDL-7, no. 7, p. 567 (1986) both incorporated by reference herein).
A back-contact Si solar cell such as the Sinton et al. design requires relatively complicated and costly fabrication, generally associated with the fabrication of integrated circuits. These processes include separate p-type and n-type diffusions (each requiring masking), alignment of the negative contact metal with respect to the positive contact metal using photolithography, and deposition of a multi-layer contact metal system by evaporation or sputtering, which requires a vacuum system. Thus, although a back-contact structure has significant advantages over a conventional front-contact structure, its implementation can be expensive.